The present invention relates to an apparatus for supplying high power electric loads operated in a pluse-like manner, especially for X-ray equipment and optionally for controlling thereof.
In engineering it is frequently encountered that for the normal operation of a high power electric load, short lasting energy is required, in which the long term average power is significantly lower than its peak value corresponding to the operational cycles. In this case, either an intermediate buffer energy store is used with sufficient storage capacity for covering the maximum power consumption, or an electrical network is established with dimensions in accordance with the maximum value of the load.
As an intermediate buffer store usually a battery or a capacitor plant is applied. A typical example for the battery application is the starting of a motor vehicle in which the high power required for the starting is supplied in expense of the energy loaded in previously, during a longer period of time at a lower power level. Capacitors are used as buffers frequently for high power discharge applications and for many other areas.
When such intermediate energy stores are used, operational current and voltage values are provided for the load, or the voltage and power demand of the load are adjusted to meet the existing energy supplying possibilities. This matching operation can be solved in many areas of engineering. There are certain applications, however, where the usual intermediate energy stores are not capable of providing proper voltage and current values required for the normal operation of the load, although their capacity would be sufficient to cover peak power consumption. Typically such application is the X-ray technique that will be discussed in detail for its great significance. However, the special problems discussed in connection with X-ray technique can also be valid in other fields of applications. Therefore, the technical background of the present invention cannot be limited to the X-ray technique since each application with similar problems can be involved.
It is widely known that the generation of X-rays, e.g. for making roentgenograms, requires high power electric energy with short duration. As an example, when utilizing the limit load of advanced X-ray tubes, 100-150 kW power is required for a duration of about 0.1 second. During the exposure this power is provided with precise adjusted voltage and current values. The high voltage of the X-ray tube ranges between 50 and 150 kV.
It is also known that the switching of high voltage circuits is a very difficult operation, due to the hazard of arc forming. It is practically impossible to adjust exposure times of some hundredths or tenths of a second by means of high voltage switches, due to their large size and inertia and because of the increased risk of arc forming. The switching on of high voltage levels can be accomplished by means of high voltage switching tubes, i.e. triodes, tetrodes or grid controlled X-ray tubes. The application of such tubes is limited, however, because of their high costs, the large space they need and the complicated way of their control. Therefore in X-ray technique, the switching of the high voltage transformers is generally carried out at the primary side.
The application of high voltage transformers excludes the use of the conventional intermediate buffer stores, since both the battery and the capacitor provide a direct current voltage being inappropriate for feeding directly the X-ray transformer. Although there exist circuits which generate alternative current with a given frequency from a direct current voltage, their application would give rise to further problems due to the usual power levels and short times of operation required in X-ray technique.
For the sake of completeness, there is known portable X-ray equipment that uses capacitors for storing the electrical energy required for field operation. These capacitors are charged through a longer period from an electric network which has a lower power-supplying ability. Then by means of appropriate change-over switches, the capacitors are interconnected (e.g. parallel-series conversion) to supply the high voltage required for operating the X-ray tube. These arrangements use complicated and expensive switches and have the disadvantage that during the discharging process the voltage of the X-ray tube continuously decreases and the X-ray quality rapidly changes. At such high tensions voltage stabilizator circuits cannot be applied.
On account of these facts, the second of the alternatives referred to above is used in X-ray technique, i.e. the electric network which feeds the X-ray equipment is dimensioned to the maximum power consumption of the equipment.
The internal resistance of a main network designed to feed advanced X-ray equipment should be very low, in the order of 0.1 ohm, so that the voltage drop occurring when the X-ray tube is switched on is tolerable and compensatable. A further difficulty is caused by the voltage fluctuation of the power supply which can have an unfavorable effect on the quality of the roentgenogram if it occurs during the preparation and completion of the exposure.
The efficiency of utilization of such a network dimensioned for power peak values is very poor and its construction is connected with high investment costs. In the majority of hospitals a separate power supply system is provided for feeding the X-ray equipment, so that the load peaks in the normal municipal energy distributing network cannot cause voltage drops in the special network that feeds the X-ray equipment. The establishment of such a separate power supply system requires significant investments, which may hamper the application of X-ray equipment and at the same time the possibilities in X-ray technique are unfavorably influenced by the imperfect supplying network.
The problem of power supply is still more acute in the case of portable X-ray equipment used in hospital wards, in intensive care units and in operating theaters. In such places, the available energy network can usually provide only low power with insufficient stable parameters (voltage, network resistance). Therefore the quality of spot-made roentgenograms taken by means of a small power X-ray equipment is not satisfactory.
Another problem is the feeding of high-power X-ray equipment which is steadily installed in operating rooms for artery catheterization and angiography and which make exposure-series during these examinations. In such rooms, for the safety of the patients, i.e. for their protection against electrical shock, all electric equipment must be fed by a network separated from the normal mains by a medical isolating transformer in accordance with the recommendations of international standards (IEC TC 62A). Since it is practically impossible to operate X-ray equipment from a separate network isolated as mentioned above because of the high power consumption, there is no other alternative but to feed it from the normal mains. The safety principles to be applied in these rooms are, however, violated by this compromise to such an extent that their efficiency becomes doubtful.
The feeding of X-ray equipment is especially problematic in disaster and emergency situations when the normal mains is generally not available. In such cases the only solution is to apply motor driven generators with high power capacity. This emergency power supply is, however, not capable of meeting the requirements of X-ray equipment.
The fact that the perfect feeding of high power X-ray equipment operating in a pulse-like manner could not have been solved by means of intermediate buffer stores has been causing problems not only in the previously discussed overdimensioning of the energy supplying system. For the above reasons it is not advisable to switch over high voltages. Therefore it is widely accepted to switch high voltage transformers of X-ray equipment at the primary side. It is due to this way of switching that both the transformer and the switches of the X-ray equipment are to be designed for the maximum power demand of the equipment. But even in case of switching at the primary side, difficulties arise when power peaks are at about 100-150 kW for as short a duration as for example some milliseconds. Because of the requirement that the time of exposure be kept precisely at the adjusted values, the necessary switching operations are carried out by means of multistage biased high power magnetic switches. In case of high power exposures of such a short duration it is also important that the voltage of the alternative current network at the moment of switching be at its minimum value. Therefore the control units of advanced X-ray equipment detect the zero crossing instances of the supplying AC network and enable the exposure only at corresponding discrete moments.
It is clear from the above mentioned that there exists a need for solving the problem of intermediate energy storage which takes into consideration the specific properties of X-ray equipment in such a way by which the pulse-like energy required for the exposure can be obtained in expense of an average power consumption of longer duration, the need for using transformers can be eliminated and the equipment can be controlled at small power levels being several orders of magnitude less than the maximum power level.
It is characteristic of the existing X-ray technique that in case of fluoroscopy the patient is irradiated with a given constant radiation intensity and the rays are viewed on a fluoroscopic screen. In case of conventionally used radiation intensities which are low enough not to be harmful to the patient, the light intensity of the image on the screen is so low that it can be viewed only in a dark room. It is known that at such low light intensities the cones of the human eye still cannot perceive light information and the light perception is provided by the rods which have much worse image resolution properties. From the aspect of the image evaluation, an increased light intensity might be of great importance, if this not connected with an increased radiation dose.
The primary object of the present invention is to provide an apparatus which is capable of supplying high-power electric loads operated in a pulse-like manner in expense of a power consumption which is substantially lower than the pulse power of said loads.
Another object of the invention is to provide an apparatus which, in addition to satisfying the aforesaid primary object, can also control the high-power electric loads and can keep their operational voltage and current values at predetermined levels.
A further object of the invention is to provide an apparatus capable of supplying and controlling X-ray equipment without the above-summarized shortcomings of the conventional X-ray technique.
Yet another object of the invention is to provide a method for increasing the light intensity of the image displayed on the fluoroscopic screen during X-ray fluoroscopy.